U.S. Pat. No. 4,164,648 (hereinafter referred to as “HP TIA,” where “TIA” is an abbreviation for “time interval analyzer”) describes an approach to time interval measurement using three analog phase locked loops. Additionally, this patent identifies the shortcomings of a two-ring oscillator time vernier approach, described for example in U.S. Pat. No. 6,295,315 and PCT International Patent WO 01/69328 A2 (hereinafter referred to as “traditional”), currently used almost exclusively. In accordance with HP TIA, all loops are locked to a common reference frequency. Two of the loops differ slightly from a third loop, referred to as the “reference.” Implementation is almost exclusively via analog circuitry.
Using analog PLL circuit techniques of HP TIA, three oscillators are locked to a system clock. One is a reference signal oscillator, and the remaining two are measurement oscillators. The measurement oscillators are locked to a frequency that differs from the reference signal by 1+1/N or 1−1/N, where N is an integer. Given an arbitrary phase and frequency start of a measurement oscillator, the phase of the measurement signal gains (or loses) until it matches the phase of the reference signal. Phase alignment requires a number of reference signal clock cycles, wherein the number of cycles is determined by the value of N.
The traditional circuit approach uses only two free running ring oscillators, whose slightly differing frequencies are calibrated at the start of the circuit operation, usually just after reset. The phase measurement is made using a measurement edge to start each oscillator and, like the HP TIA circuit above, counts how many clock cycles elapse for the phases to align. The free running oscillators, even if initially calibrated accurately, are unlikely to retain calibration at the time of measurement, if measurement occurs significantly later. The traditional circuits leave the oscillators off before a measurement, such that the precise dynamic bias conditions that existed during calibration will not apply during the measurement, if the oscillators are off for any significant period of time. If the oscillators are left on, the elapsed time between calibration and measurement may allow for a drift in the frequencies in either or both of the oscillators.